select the attribute II, compare the atrribute

i have, in one XML file this:
<id_mensajesforos seleccionado="si">46</id_mensajesforos>

how can i determine the attribute value?
for this i am making this:
xsl:choose>
<xsl:when test="id_mensajesforos[(_at_)seleccionado='si']">
s </xsl:when>
<xsl:otherwise>n</xsl:otherwise>
</xsl:choose>

but it doesn't run well.
can you help me for knowing if the seleccionado attribute value is "si or
no?
thanks



 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list